Experimental study on the bubble generation characteristics
for an venturi type bubble generator
Junlian Yin
a
, Jingjing Li
a
, Hua Li
b
, Wei Liu
b
, Dezhong Wang
a,
⇑
a
School of Mechanical Engineering, Shanghai Jiao Tong University, 200240, China
b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, China
article info
Article history:
Received 5 December 2014
Received in revised form 18 May 2015
Accepted 18 May 2015
Available online 7 August 2015
Keywords:
Bubbly flow
Bubble generator
Bubble size
Two-phase flow
abstract
Bubble generation plays a significant role for mass transfer. An experimental study was performed in a
Venturi type bubble generator to evaluate the development of the bubble size distribution in the turbu-
lence flow of an air–water system. As the air stream enters into the flowing water stream through small
feedholes, it breaks into bubbles in the diverging section of the venturi tube. Based on the results of
Kress’s formulation on turbulence dissipation rate, the bubble size correlation with the Reynolds number
and the surface tension coefficient was derived. For purpose of validation, the bubbly flow under various
Re and gas volume ratios was measured by visualization technique. The experimental data proved that
the volume averaged bubble diameter has a 1 power dependence on the Re, which agrees well with
the theoretical derivation. Also, it is found that the bubble size is a linear function of the gas volume ratio
and a revised formulation which includes the gas volume ratio effect was proposed.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The Generation-IV International Forum for the development of
new nuclear energy systems has established a set of goals as
research directions for nuclear systems: enhanced safety and
reliability reduced waste generation, effective use of uranium or
thorium ores, resistance to proliferation, improved economic com-
petitiveness. The Molten Salt Reactor (MSR) is one of the candi-
dates retained by Generation IV. MSRs are based on a liquid fuel,
so that their technology is fundamentally different from the solid
fuel technologies currently in use. One of the advantages is that
the fuel can be burned up completely due to the gas removal
system application, which can separate the fission gases such as
135
Xe and
85
Ke, can be realized.
For example, an off-gas system was designed in the Molten Salt
Breeding Reactor (MSBR) by ORNL [1]. The principle of the off-gas
system is to continuously inject helium bubbles into the salt
stream to a gas volume fraction of 0.2–1.0% at the reactor core.
The
135
Xe would transfer to the helium bubbles by turbulent diffu-
sion and would be stripped from the salt by a gas–liquid separator.
The core for the gas removal process is bubbling mass transfer,
which is widely used in chemical engineering [2]. Thus, bubble
generator is a must for bubbling mass transfer and various types
of bubble generators have been developed. For example,
Sadatomi [3] developed a micro-bubble generator with a spherical
body in a flowing water tube. Ohl [4] presented a bubble generator
for single bubbles of controllable size by injecting a short burst of
gas into a liquid channel flow at various time periods.
Researchers from ORNL found that the Venturi bubble generator
is suitable for its simplicity and feasibility. From the point of view
of mass transfer, the bubbles generated must be controlled to be
smaller than 0.5 mm. Also, considering the pipe installation, the
bubbles cannot be coalesced at the upstream of the gas–liquid
separator. To do this, it is of importance to correlate the bubble size
and its distribution in space with the operational parameters such
as the Reynolds number, and the gas volume ratio.
The bubble breakup dynamics was numerically studied by
many, such as Qian [5], Chen [6] and Zhang [7], however, the bub-
ble statistic can only be acquired by measurement. Gabbard [8]
found in his experimental studies the volume averaged bubble size
presents a 3/5 power dependence on the surface tension and a
4/5 power dependence on the Re. Also, Gabbard posed a theoret-
ical formulation for bubble size prediction, which indicates the
power index for Re is not 4/5 but 6/5. The reason for this dis-
crepancy between experimental data and theoretical prediction
explained by Gabbard is induced by the inappropriate formulation
of the turbulence dissipation rate. A pertinent study by Fujiwara
[9,10] on the bubble breakup phenomenon in a Venturi tube
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.076
0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.
⇑
Corresponding author. Tel./fax: +86 21 34205499.
E-mail address: dzwang@sjtu.edu.cn (D. Wang).
International Journal of Heat and Mass Transfer 91 (2015) 218–224
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt